Thin film field-effect transistors (FETs) are the basic building blocks in the fields of microelectronics. These FETs have three electrodes (e.g., a source electrode, a drain electrode, and a gate electrode), an insulating layer, and a semi-conducting layer. The FET functions as a capacitor where the semi-conducting layer is a conducting channel between two electrodes, i.e., the source electrode and the drain electrode. The density of charge carriers in the channel is Adjusted by a voltage applied to the gate electrode, so that the electric charge flow between the source and the drain electrodes can be controlled by the voltage applied to the gate electrode.
Recently increasingly interest has focused on the development of FETs using organic semi-conducting materials. When using the organic semi-conducting materials in FETs, electronic devices can be manufactured by means of a printing method, such as screen-printing, ink-jet printing, or micro-contact printing. In addition, these materials can be processed at a much lower substrate temperature and with little or no vacuum involved, as compared with the typical inorganic semi-conducting materials. Therefore, electronic devices, including FETs, which use organic semi-conducting materials, can be easier and less costly to produce as compared with those using inorganic semi-conducting materials.
Different types of organic materials such as small molecule, polymers and oligomers have been studied for the uses as organic semi-conducting materials in FETs since the 1980s. With concerted effort in this area, the performance of the organic FET has improved from 10−5 cm2/Vs to 1 cm2/Vs in terms of charge carrier mobility in a FET (J. M. Shaw, P. F. Seidler, IBM J. Res. & Dev., Vol. 45, 3 (2001)). The performance of the organic thin film transistor is now in the level of comparable to that of an amorphous silicon transistor, so that organic thin film transistors can be applied to E-papers, smart cards and display devices.
Important electronic devices, which can be manufactured using semi-conducting organic materials, include organic light emitting diodes, organic solar cells, and organic transistors. In these devices, the electrical contact between the semi-conducting organic materials and the electrodes is crucial to improving the performance of these devices. For example, charge-carrier injection layers, such as hole-injection and electron-injection layers, are interposed between semi-conducting layers and electrodes to improve the performance of organic light emitting diodes. Even though the operation mode of the organic transistor is different from that of the organic light emitting diode, electrical contact between the semi-conducting layer and the source and drain electrodes has a profound effect upon the performance of the organic transistor.
It has been reported that the performance of the organic transistor depends upon the source/drain materials (Y. Y. Lin et al. Materials Research Society Symposium Proceedings (1996), 413(Electrical, Optical, and Magnetic Properties of Organic Solid State Materials III), 413-418. CODEN: MRSPDH ISSN: 0272-9172). According to this report, metals with high work functions (for example, Pd, Pt, and Au) showed excellent performance while metals (for example, Al) with relatively low work functions showed a significantly degraded performance. Therefore, metals with high work functions such as gold have been used for the source/drain electrodes in most organic transistors.
However, such the metals having high work functions, which are noble metals, are expensive and hard to process using industrial methods, thus restricting their application processes and application structures in the organic transistors.